Clinical Aspects

Boys affected with choroideremia complain of difficulty seeing at night in the first decade of life and become aware of loss of peripheral vision by their teens and twenties. By the fourth decade, male individuals with choroideremia have a significantly constricted peripheral visual field. Suitable imaging techniques are required to observe clinical manifestations and choroideremia disease progression, and may have the potential to monitor treatment efficacy. Firstly, optical coherence tomographic (OCT) imaging provides high-resolution in vivo imaging to define progressive retinal changes and reveals areas of hypopigmentation of the RPE and thinning throughout the fundus in the early phase of choroideremia. Retinal thickening occurs subsequently with normal laminae, possibly attributable to Müller cell activation and hypertrophy creating interlaminar bridges. Later phases show typical shortening of the inner and outer segments, reduced thickness of the outer nuclear layer, and depigmentation of the RPE. Areas of chorioretinal atrophy become evident over time with loss of choriocapillaris, exposure of choroidal vessels, and loss of the RPE beyond the macula ( Figure ).

Retinal changes associated with choroideremia. (Top left) Fundus photograph of the right eye of a 52-year-old male affected with choroideremia showing large atrophic areas of the retina and choroid. (Top right) Infrared image of the retina depicts the orientation of the optical coherence tomographic scan showing a small patch of remaining choroid, retinal pigment epithelium (RPE), and photoreceptor elements with adjacent retinal tubulation. (Bottom left) Fundus photograph of the right eye of a 5-year-old boy with fine punctate appearance of the RPE and small depigmented lesions temporal to the fovea. (Bottom middle) Enlarged fundus photograph of the left eye of an asymptomatic female carrier with small pigmented spots in the midperiphery of the retina. (Bottom right) Fundus autofluorescence of the left eye of a female carrier showing heterogeneity of the autofluorescence.

Secondly, fundus autofluorescence (FAF) provides a noninvasive approach to imaging of the RPE. In male subjects affected with choroideremia, FAF allows specific delineation of the remaining RPE and observations of the progressive shrinkage of the central RPE island at rates that can be calculated for individual patients. The rate of loss is greater in younger than older patients. By combining FAF with OCT, the loss of both RPE and photoreceptors can be followed. The observation of early photoreceptor loss in the vicinity of disappearing RPE suggests a concomitant pathologic process occurring in both photoreceptors and RPE cells.

With a milder phenotype, female carriers can be difficult to recognize in childhood. However, detection is aided by FAF imaging, which reveals a pattern of mixed hypoautofluorescent spots, which could correspond to atrophy of the RPE, and hyperautofluorescent spots, which could signal dysfunction of photoreceptors and/or accumulation of lipofuscin in RPE cells ( Figure ). Occasionally, female carriers are severely affected and exhibit a phenotype not unlike male subjects with choroideremia. Although lyonization has traditionally explained the clinical findings in affected female subjects, a plausible explanation may also be insufficiency of REP activity. By inference, the combination of lyonization and the carrier state may deplete tissues such as the RPE of REP-1 and manifest as disordered cellular function. Symptomatic choroideremia carriers may result from skewed lyonization or variability in X inactivation.

Further dissection of the carrier phenotype in choroideremia can be performed by combining fundus-controlled perimetry, multifocal electroretinography (mfERG), 2-color threshold perimetry, spectral-domain OCT, and FAF. Two-color threshold perimetry of choroideremia carriers demonstrates that most have a more pronounced loss of rods, but some have a more prominent loss of cones. Whether cone loss is secondary to the RPE impairment or to a primary cone dysfunction is not known. mfERG superimposed on FAF images shows no strict correlations between abnormal FAF spots and changes in mfERG responses.

Genetics

The CHM gene consists of 15 exons, spans 186 kilobases (kb), and encodes a polypeptide of 653 amino acids. Mutations of the CHM gene that lead to premature termination of translation of REP-1 or the absence of REP-1 include sizeable deletions (20%), point mutations (60%), intronic mutations (10%) and unknown causes (10%) Using mRNA analysis, a deep intronic mutation has been detected in intron 4, resulting in a pseudoexon containing a stop codon, and in intron 12, leading to exon skipping. An L1-retrotransposon insertion that occurred de novo in a choroideremia patient has also been identified. The identification of CHM mutations is important for genetic counseling, accurate clinical management, and on-going therapies.

An autosomal homologue of CHM designated CHML , for choroideremia-like, has also been identified. The gene product of CHML is REP-2, which has an overlapping but not identical function to REP-1. It is believed that REP-2 compensates for a deficiency of REP-1 in all human tissues, except the eye.

Recently, several databases have been developed for CHM genetic variations, including LOVD-CHM (Leiden Open Variation Database; http://www.lovd.nl/CHM ), an open-source tool that provides a complete and accurate repository of published mutations occurring in CHM . Currently, it contains 127 published CHM variants, 259 entries, and 36 PubMed references. A second LOVD-CHM database dedicated to published mutations is available at http://ngrl.manchester.ac.uk/LOVDv.2.0/home.php?select_db=CHM .

Gene Therapy

The field of gene therapy for blinding disorders of the retina is at a turning point because of recent advances in proof-of-concept studies, improvements in vectors and technology, and the first-in-human experiences. The summit was attained in 2008 by the first phase I/II clinical trials for an autosomal recessive, congenital form of severe blindness, Leber congenital amaurosis (LCA), caused by mutations in RPE65 , a gene specifically expressed in the RPE. The encouraging results and safety data collected from these and additional trials have paved the way to retinal gene transfer for other monogenic diseases, such as choroideremia.

Many of the difficulties associated with designing gene therapy for choroideremia surround the choice of vector. The first issue is that the choroid is the most active blood supply in the body. Thus, any vector administered to the choroid will be flushed away instantly. It is not possible to stop blood flow for a limited time without damage; therefore, it is important to resolve the enigma of whether choroidal atrophy (which appears early in disease pathogenesis) is secondary to RPE degeneration. The second consideration is which cell types to target. The consensus is that both RPE and photoreceptors should be targeted. A third issue is the promoter to use. The clinical RPE65 trials with a cell-specific promoter have shown the least amelioration in patients, suggesting that a strong promoter is required for sufficient expression of exogenous protein. This is reassuring, as a ubiquitous promoter is likely needed for choroideremia, since 2 tissue types must be targeted. The final consideration is the intellectual property associated with the chosen serotype. This can cause a significant hurdle by blocking the administration of the final drug to patients.